@Aurélien4 Short answer: no, you can't.
If you want to unlock the dates that have been selected in a Request, and you haven't made a decision about the guest, the only thing you can do is decline the request. You'll still have the opportunity to send them a Special Offer or let them re-submit a Request if you're satisfied with the correspondence later on.
Bear in mind, this adds a lot of hassle and confusion to the booking process for the guest. If you find yourself having to engage in such lengthy discussions before making a decision, it's worth pinning down what exactly your requirements are in order to feel comfortable with a booking, so you can get them all into your initial message.